BUG: qhull: fix potential infinite loop in RidgeIter2D 

For certain triangulations, RidgeIter2D could enter an infinite loop due to degenerate simplices.  This commit replaces the RidgeIter2D by an explicit list of neighboring vertices.

An explicit list is also more useful for higher-dimensional data. (E.g. gradient estimation and N-dimensional data smoothing can be implemented later on.)

Fixes gh-2359